The present invention generally relates to power cords in place on electronic systems and instruments, and more particularly to instrument and system designs capable of minimizing unwanted high frequency noise signals.
The present invention applies to any electrical or electronic system, such as a computer system. To those skilled in the art of computer hardware technology, it is understood that computer systems include several general components or sub-systems, including a power cord, a line filter, a power source sub-system, and a load sub-system, as such as computer sub-system connected to a power grid.
Electrical systems are required to pass various FCC tests and/or regulations prior to introducing the system to the general public. One particular test of interest is the conducted emissions test, which determines the amount of noise signals conducting on a power cord in the direction toward a power grid from a power source sub-system and a load sub-system. If the noise signals exceed a threshold determined by the FCC for the particular system being tested, the system fails the test. Currently, the conducted emissions test of the FCC tests systems at frequencies between 10 kilohertz and 30 megahertz. Noise signals conducted at frequencies greater than 30 megahertz are not determined, and therefore, not regulated.
A second test of interest is the radiated emissions test, which determines the amount of noise signals radiated from the system including the power cord during operation. If the noise signals radiating from the power cord exceed a threshold determination by the FCC for the particular system being tested, the system fails the test. Currently, the radiated emissions tests of the FCC test systems at frequencies between 30 megahertz and the larger of either one gigahertz or 5 times the maximum system frequency. Noise signals radiated at frequencies outside of this range are not determined, and therefore, not regulated.
As technology advances, the system maximum frequency of electrical systems consistently increases. For example, present and future electrical systems will include maximum system frequencies up to the low gigahertz range. Conventional line filters used in electrical systems to prevent conducted emissions (noise signals) at low range frequencies do not adequately work at these elevated frequency ranges, and tend to permit radiated noise signals from the power cord.
Conventional line filter designs do not adequately work at elevated frequencies due to impedance elements (e.g., capacitors and inductors) that typically have various parasitic components associated with them. For example, at low range frequencies, an inductor has parasitic capacitances which do not significantly impact the amount of noise signals radiated from a particular system. Conversely, at elevated frequency ranges, the parasitic capacitances act as coupling devices which circumvent the effects of the inductor. The purpose of an inductor is to provide a high-impedance, low-conductance path for high-frequency emissions. However, the parasitic capacitances provide a high frequency path through the inductor, which is opposite of a desired effect of an inductor, which is to provide a high impedance, low conductance path for high frequency emissions. Thus, at elevated frequencies, an inductor of a conventional line filter design acts more like a capacitor, rather than an inductor.
Similarly, a capacitor of a conventional line filter resembles an inductor at elevated frequencies, rather than a capacitor. More specifically, a capacitor within a conventional line filter design has associated parasitic inductances. The parasitic inductances become the dominant impedance at elevated frequencies. Thus, capacitors which normally channel unwanted emissions to earth ground through an enclosure, instead look like a high impedance path and do not provide the desired path for unwanted radiated emissions to earth ground.
Therefore, a filter system designed to minimize radiated noise signals, simply passes noise signals at elevated frequencies, thereby contributing to an inefficient overall filter system.
Thus, there is a need for a line filter assembly and design which will minimize unwanted noise signals at elevated frequencies, such as at frequencies up to the mid-gigahertz range. The assembly and design must meet this criterion without significant cost or expense. Significant cost or expense would render the assembly and design impractical. It is desirous to provide a system and design which is reliable, inexpensive in its components, and inexpensive to implement.
The line filter assembly of the present invention comprises a high frequency filter which includes a PC board having first and second sides. A first pad covered with an electrically conducting material is positioned such that the electrically conducting material is adjacent to the first side of the PC board. A first electrically conducting brace is in electrical connection with the electrically conducting material covering the first pad and in electrical connection with an enclosure of the line filter assembly representing earth ground. First and second electrically conducting plates are fabricated on the second side of the PC board. The first electrically conducting plate is indirectly connected to a power line of the power cord, while the second electrically conducting plate is indirectly connected to a second power line of the power cord. A dielectric film is positioned adjacent to the first and second electrically conducting plates. A second pad covered with an electrically conducting material is positioned such that the electrically conducting material is adjacent to the dielectric film. A second electrically conducting brace is an electrical connection with the electrically conducting material covering the second pad and in electrical connection with the enclosure of the line filter assembly.
In another embodiment of the invention, the line filter assembly previously described is used in a larger overall system capable of diverting a wider range of emissions between a power source sub-system and a power grid. A primary filter sub-section is electrically connected to the power source sub-system. A high frequency filter is electrically connected between the primary filter sub-section and a secondary filter sub-section. The secondary filter sub-system is electrically connected to the power grid.
In yet another embodiment, a high frequency common mode ferrite is electrically connected between the primary filter sub-section and the high frequency filter. Further, the first and second pads of the line filter assembly are each formed from a foam rubber pad, while the first and second electrically conducting plates are each formed from a sheet of copper.
In still yet another embodiment, the first electrically conducting brace further comprises a substantially L-shaped metal brace. Further, the dielectric film of the line filter assembly is fabricated from a Mylar(copyright) film, while the PC board is formed from a fiberglass composition. Additionally, the PC board has a capacitance in the range of 5-25 picofarrads per square inch, while the dielectric film has a capacitance in the range of 25-100 picofarrads per square inch.
The present invention is a simply (easy to assembly) design utilizing inexpensive components to minimize unwanted noise signals radiated on a power cord at elevated frequencies. The present invention modifies a conventional design for minimizing unwanted noise signals at low frequencies such that unwanted noise signals at any frequency into the mid-gigahertz range is transmitted to ground.